The use of magnetic inks in currency already allows vending machines to verify the authenticity of bank notes. But new research shows that metal detectors at security checkpoints could detect and count large stacks of notes from a distance for catching smugglers.

Physicists Christopher Fuller and Antao Chen, from the University of Washington in Seattle, found that ordinary handheld metal detectors were able to recognize the magnetic ink in a single dollar bill from just over an inch away. And as multiple bills were stacked, the size of their magnetic field also increased, allowing the researchers to not only detect them from a greater distance, but also count how many there were.

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While the researchers intend to present their work at an upcoming Defense, Security and Sensing conference, there's still a lot of work to be done before detectors could be smart enough to find a stash of smuggled bills. At the moment determining the dollar value is impossible since different denominations all use the same amount of magnetic ink. And metal detectors would have to be vastly improved so they could automatically differentiate the magnetic ink from other sources. [IOP Science via New Scientist]